Computer peripheral devices, such as Local Area Network (LAN) or modem Personal Computer (PC) Cards perform input/output (I/O) communications functions in portable computers. These computer peripheral devices, generate some level of electromagnetic interference (EMI) emissions during data transmission. The permissible level of EMI is strictly regulated by national or regional regulatory bodies. For instance, in the U.S., any portable computer or handheld computer used in a home environment may only use a PC card which meets FCC Class B standards for EMI. In Europe, the corresponding requirement is the CE standard.
The ability of a LAN or modem I/O card to meet the proscribed EMI levels is directly related to effectiveness with which ground signals are dissipated from the I/O card during active LAN or modem data transmissions. In PC cards, the PCMCIA (personal computer memory card international association) industry standard allocates and designates 4 pins out of a total of 68 interface pins as ground signal pins.
In one prior art approach, even 4 pins prove inadequate to sufficiently dissipate the ground signals, thereby requiring manufacturers to add a bulky ferrite core externally to the I/O cable used in conjunction with the PC Card, to limit EMI to the required levels. This method adds cost and bulk to the PC Card, and is aesthetically displeasing to the end-user.
In another prior art approach, in addition to using the 4 pins available for ground signals, some manufacturers use a complicated arrangement of individual and separate ground clips, which are then assembled onto the card frame to provide additional grounding paths from the I/O card through the host computer. Assembly of these clips onto the card frame requires time-consuming, multi-step manual operations. Such extensive manual labor results in a more costly finished product. The manual assembly process is also prone to wide variation in skill level among individual manual assemblers, and results in inconsistent quality of finished product, as regards effectiveness in meeting regulatory EMI limits.
In yet another attempt to control the EMI levels generated in PC Cards, other manufacturers, in addition to using the 4 pins designated, use metal tabs on the top surface of the 15 pin I/O connector at the I/O end of the card. This arrangement is intended to provide an additional grounding path from the card circuitry, through the metal tabs being in direct contact with the metal covers of the card, which in turn are electrically connected to a grounding signal path on the host computer. However, the location of such metal tabs at that I/O end of the card results in the tabs being continually subjected to compressive forces during user insertion and removal of the card into the host computer PC Card slot. These repeated cycles of compressive forces result in the metal tabs being permanently deformed after only a few hundred cycles of usage, so that they are no longer in contact with the metal cover of the card, and this feature ceases to function as a grounding path. Thus, a PC card which might meet EMI regulatory requirements upon shipment of the product to a customer, gets progressively worse when used as intended.
As handheld computers, which are even smaller than portable computers, become more widespread, users will increasingly have a need for correspondingly smaller I/O cards to handle LAN and modem communications functions. Currently, the Compact Flash Card is a relatively newer form factor which is about one-third the overall size of a PC card. The Compact Flash Association has allocated and designated 2 pins out of a total of 50 interface pins as ground signal pins. For LAN or modem I/O cards having a Compact Flash form factor, it will be practically impossible, or extremely difficult at least to meet FCC Class B and CE regulatory requirements during active data transmission, when only the allocated 2 pins are available as ground signal pins.
Therefore, a need exists for a computer communications I/O device structure that will effectively dissipate ground signals while not requiring more than two interface pins dedicated to ground signals. A further need exists for a computer communications I/O device structure which effectively dissipates ground signals wherein the device does not require costly, labor intensive, manual operations which are likely to result in widely varying finished product quality, in terms of EMI effectiveness. Still another need exists for a computer communications I/O device structure which effectively dissipates ground signals, and in which the effectiveness of any ground signal paths used does not degrade relatively rapidly with time, and wherein the device does not violate EMI regulatory requirements when used as intended.